linux-kernel - Re: [PATCH 1/4] x86/coco: Add a new CC attribute to unify cache flush during kexec

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Message-ID: <20240220142812.GBZdS2_HJEA50F8RZD@fat_crate.local>
Date: Tue, 20 Feb 2024 15:28:12 +0100
From: Borislav Petkov <bp@...en8.de>
To: Tom Lendacky <thomas.lendacky@....com>
Cc: "Huang, Kai" <kai.huang@...el.com>, linux-kernel@...r.kernel.org,
	x86@...nel.org, dave.hansen@...el.com,
	kirill.shutemov@...ux.intel.com, tglx@...utronix.de,
	mingo@...hat.com, hpa@...or.com, luto@...nel.org,
	peterz@...radead.org, chao.gao@...el.com, bhe@...hat.com,
	nik.borisov@...e.com, pbonzini@...hat.com
Subject: Re: [PATCH 1/4] x86/coco: Add a new CC attribute to unify cache
 flush during kexec

On Mon, Feb 19, 2024 at 04:09:47PM -0600, Tom Lendacky wrote:
> That's why the '!(sev_status & MSR_AMD64_SEV_ENABLED)' works here.

I would've never figured that out just from staring at the test. :-\

> Basically, if you are bare-metal, it will return true. And it will only
> return true for machines that support SME and have the
> MSR_AMD64_SYSCFG_MEM_ENCRYPT bit set in SYS_CFG MSR because of where the
> 'cc_vendor = CC_VENDOR_AMD' assignment is. However, if you move the
> 'cc_vendor = CC_VENDOR_AMD' to before the if statement, then you will have
> the WBINVD called for any machine that supports SME, even if SME is not
> possible because the proper bit in the SYS_CFG MSR hasn't been set.
> 
> I know what I'm trying to say, let me know if it is making sense...

Yah, thanks for taking the time to explain.

Here's an even more radical idea:

Why not do WBINVD *unconditionally* on the CPU down path?

- it is the opposite of a fast path, i.e., no one cares

- it'll take care of every possible configuration without ugly logic

- it wouldn't hurt to have the caches nice and coherent before going
  down

Hmmm.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette